Referring to FIG. 1 of the drawings, the reference numeral 100 generally designates a conventional headphone subsystem. With the headphone subsystem 100, a headphone jack 110 is inserted into a receptacle 112 to allow external components (such as speakers) to interact with circuitry in a device (such as a portable media player). Typically, headphone jacks (such as jack 110) use four pins (labeled as 1, 2, 3, and 4) that allow a user to provide input (through a microphone, for example) and receive an output (through speakers, for example) with one receptacle and one jack. An example of this type of subsystem 100 is a hands-free microphone/speaker commonly used with mobile phones.
In operation, the subsystem 100 is able to discern which of the pins (1, 2, 3, or 4) of jack 110 is ground and is able to discern the direction of data. To accomplish this, subsystem 100 is generally comprised of amplifiers 102, 104, 106, and 108, controller 114, switch network 116, and capacitors C1 and C2. Amplifier 102 and 104 provide output voltages VL and VR to pins 1 and 2 of jack 110 through receptacle 112 so as to provide outputs for left and right speakers, for example. Amplifier 108 (which is coupled to pins 3 and 4 through capacitors C1 and C2) operates as a preamplifier for a microphone, for example. Amplifier 106 and resistor R1 provide a bias voltage to switch network 116. Generally, pins 3 and 4 can be either ground or an input (i.e., microphone input), which is a reason for the switch network 116. Controller 114 is able to detect which of the pins 3 or 4 from jack 110 is ground and is able to control switches S1, S2, S5 and S6 accordingly. For example, when pin 4 of jack 110 is detected as being ground, switch S6 is actuated to coupled pin 4 to ground, and switch S1 is actuated to couple pin 3 of jack 110 to resistor R1.
Turning to FIG. 2, a simplified configuration for amplifiers 102 and 104 can be seen. As can be seen, speakers 118 and 120 share ground with a microphone. In operation, speaker 118 is supposed to receive voltage VL, while speaker is supposed to receive voltage VR. However, if voltage VR is 0, then the voltage across speaker 120 is the voltage across switch S2 or S1, which is proportional to voltage VL because switch S2 or S1 is coupled in series with speaker 118. Thus, there is a voltage drop across speaker 120 that is proportional to voltage VL, which is cross-talk. The cross-talk depends on the ON-impedance of the switch S1 or S2. In particular, the cross-talk (CT) can be calculated as follows:
                    CT        =                  20          ⁢                      Log            (                                          R                SW                                                              R                  SP                                +                                  2                  ⁢                                      R                    SW                                                                        )                                              (        1        )            where RSW is the impedance of switch S1 or S2 and RSP is the impedance of the speaker (i.e., speaker 114). For example, if RSW is 0.1Ω and RSP is 16Ω, then there is cross-talk of 22 dB. Therefore, there is a need for circuitry that reduces cross-talk.